I think the only thing that Nomura has officially confirmed is that the heart-shaped moon summoned by the old man is not truly Kingdom Hearts, although I've no quote or other evidence to back that up, so take it with a grain of salt.

The only things I'm comfortable about deducing from the video are that it takes place sometime before Xehanort loses his memories and appears in Radiant Garden (mostly based on conclusions drawn from a conversation between Xigbar and Zexion in a scene from FM+) and that Mickey appears to still be in training under Yen Sid (for reasons previously mentioned).

I'd say that Clawsze's analysis makes some sense. The way Xehanort's Heartless speaks about possessing Riku's body leads one to believe it is something he has done before. Since they wear the same clothing, it is obvious that there is likely some connection between them. That aside, Terra (although I've still not seen any direct evidence that's his name--then again, I've not been looking for it either) does look strikingly similar to Xehanort (specifically the haircut and the glowing eyes). I wouldn't be surprised if somehow the old man transfers his heart to Terra so when Xehanort casts off his body, the old man's heart becomes the source of the heartless and Terra's body becomes Xemnas (which would explain why he does not remember having a heart of his own). It's purely speculation anyway.

I remember reading a theory that somehow Ventius's heart was transfered to Sora much as Kairi's heart was in Kingdom Hearts. If Roxas plays host to this heart, it would explain why Roxas looks so much like him, and why Roxas is different from the other Nobodies and is able to wield a keyblade (which should require a heart). Again, purely speculation I'm not willing to put much stock in.

It's interesting to consider, but I'm not going to pay much attention to anything until a while after the next game is announced.